Abstract
So far coastal engineering has been focused on protecting the coastline and its assets – urban, transport, industrial – from wave energy, with no interest in the exploitation of this valuable resource as a form of renewable energy.
With the advent of technology for transforming wave power into electricity, the objective of coastal engineers will be to protect the coastline and harness wave power, rather than to dissipate or reflect it back to sea – a paradigm shift in coastal engineering.
To achieve economies of scale and significant levels of production, wave energy converters (WECs) will be grouped to form wave farms, which will serve a dual purpose in erosion-prone areas: the production of carbon-free electricity and the mitigation of erosion.
Our approach to protecting the coastal zone and its assets is largely based on structures, which armour the coastline to a greater or lesser degree, often at the expense of any beaches that may have existed in the area – in particular, in the case of seawalls.
Indeed, the impact of these structures is typically large, and armoured coastlines bear little resemblance to natural coastlines. Another downside of coastal structures is that they will struggle to adapt to the impending (arguably, ongoing) climate change, which will raise sea levels and increase storminess.
This is because they are fixed to the seabed and have a certain height above the water level, or freeboard. As the sea level rises, the freeboard of the structures decreases and so does the level of protection they offer against flooding.
Considerable development in wave energy technology
In recent years, there has been considerable development in wave energy technology, which harnesses the power of ocean waves for the production of electricity. A number of wave energy converters (WECs) are being developed. For wave energy to be viable economically, WECs will be grouped in arrays, forming wave farms – much as wind turbines are grouped to form wind farms.
These wave farms will be deployed typically in intermediate waters – well clear of the surf zone, where breaking waves create an extremely harsh environment, but not in deep water either, where the mooring systems and the submarine cables to the shoreline would be too expensive.
In extracting part of the incident wave power (for conversion into electricity), the wave farms will reduce the wave power that reaches the coastline, thereby creating a sheltered area in their lee.
In a series of papers, we investigated this nearshore wave power reduction or shadow effect (Carballo and Iglesias, 2013; Veigas et al., 2014) and whether it could reduce erosion on vulnerable coastlines, where wave-driven processes are responsible for erosion (Abanades et al., 2014a, b, 2015a; Abanades et al., 2015b).
As a case study we used Perranporth – a beach in Cornwall (UK) subject to severe erosion. We considered two scenarios: with and without a wave farm. The wave farm consisted of 11 WaveCat WECs.
WaveCat is a floating, overtopping WEC designed for offshore deployment, of the terminator type (Figure 1). Laboratory tests of a WaveCat model were carried out to characterise its interaction with the wave field (Allen et al., 2016; Fernandez et al., 2012; Iglesias et al., 2009).
Effects of the wave farm on sediment transport and coastal morphodynamics
The results of these tests allowed us to compute the influence of the wave farm on wave propagation from offshore towards the beach (the shadow effect) and, consequently, the effects of the wave farm on sediment transport and coastal morphodynamics, by comparing the two scenarios (with and without the wave farm) over different periods of time.
To this end, we used state-of-the-art numerical modelling, more specifically a third-generation spectral wave model (SWAN) coupled to a coastal processes model (XBeach).
To facilitate the comparison and characterise the effects of the wave farm on the beach, we defined a number of ad hoc indicators, including the Nondimensional Erosion Reduction (NER) (Abanades et al., 2015a).
This is the percentage of erosion reduction achieved thanks to the wave farm. We found values of NER in excess of 30 per cent over periods of one and three months in the beach section directly in the lee of the wave farm. Along some critically eroding beach profiles, the landwards extreme reached by erosion was displaced 15m seawards thanks to the wave farm.
Very significant reductions in erosion
These are very significant reductions in erosion, which open up the possibility of envisaging wave farms not only as a means of producing carbon-free energy but also as a coastal management tool.
We also found that the distance between the farm and the coast and the layout of the farm play a role in its effects, and consequently in its capacity to mitigate erosion (Abanades et al., 2015a). Research is continuing on these and other aspects of the application of wave farms to coastal protection.
The advantages of wave energy farms as a coastal management tool vis-à-vis coastal structures are apparent, in particular in the case of farms consisting of floating WECs, which will arguably be the majority.
First, these farms will adapt naturally to sea level rise, unlike coastal structures fixed to the seabed. Second, their visual impact will be much lower, thanks to their offshore position and the fact that most WECs have a small freeboard – much lower than that of offshore wind turbines.
In some cases wave farms may be able to replace coastal structures entirely; in others, they may complement structures or beach renourishment schemes.
Development of robust and efficient WECs essential for wave energy to develop
The development of robust and efficient WECs is essential for wave energy to develop and, in particular, for wave farms to be used for coastal protection, and there is no denying the level of this challenge. Robust to withstand storms – in the knowledge that the ratio between the peaks and the average values in wave power is far greater than in other renewables.
Efficient to make wave energy a viable proposition from an economic point of view – and progress is being made in this regard thanks to the intensive R&D effort under way.
Incidentally, the dual function that will be fulfilled by wave farms off vulnerable coastlines will contribute to their economic viability, provided the benefits to the community are passed to the developers of the wave energy project through appropriate incentives.
Multiple policy instruments are available to this end (feed-in tariffs, tax breaks and so on), and these should be structured in such a way that the benefits for the community in terms of savings on coastal structures, reduced visual impact, or enhanced level of protection of urban properties, transport infrastructure, beaches and other natural assets (the so-called externalities) are indeed accounted for.
Wave energy not so expensive a proposition as it might appear at first
In economics parlance this is called internalising the externalities; if properly carried out, wave energy is not so expensive a proposition as it might appear at first, whereas other, conventional energy sources (for example, nuclear) are considerably more expensive that we tend to think (Astariz and Iglesias, 2015, 2016).
A few words on the technological challenge. There is no question that wave energy requires that new technology be developed to operate in a hostile environment. As with most nascent industries, the beginnings are not exempt from errors, and these have made some people, perhaps naturally, sceptical about the future of the sector.
Well, thousands of years ago, fishing from a boat at sea also required very advanced technology (for its day) operating in a hostile environment, and we can be certain that it did not happen overnight and errors were committed. But the incentives were clear – a much bigger catch than that which could be made fishing in rivers – and eventually, the technology was developed.
Nowadays, we could not imagine ourselves fishing only in rivers and neglecting the considerable resource out at sea. In the energy sector, we are still fishing only in rivers and neglecting the enormous resource in the sea. But the technology will eventually be there – not before long, at the current pace of development – and the day will come when it will seem unthinkable to not harness wave power.
In sum, wave energy is poised to become a fully fledged renewable, and the hypothesis that wave farms can serve for coastal erosion management in addition to their primary purpose of energy production is being investigated, with promising results so far.
This will mark a paradigm shift in coastal engineering. For Ireland, a country with a wave resource that is second to none, the development of wave energy may well be a strategic priority.
References
1.) Abanades, J., Greaves, D., Iglesias, G., 2014a. Coastal defence through wave farms. Coastal Engineering 91, 299-307.
2.) Abanades, J., Greaves, D., Iglesias, G., 2014b. Wave farm impact on the beach profile: A case study. Coastal Engineering 86, 36-44.
3.) Abanades, J., Greaves, D., Iglesias, G., 2015a. Coastal defence using wave farms: The role of farm-to-coast distance. Renewable Energy 75, 572-582.
4.) Abanades, J., Greaves, D., Iglesias, G., 2015b. Wave farm impact on beach modal state. Marine Geology 361, 126-135.
5.) Allen, J., Sampanis, K., Wan, J., Greaves, D., Miles, J., Iglesias, G., 2016. Laboratory Tests in the Development of WaveCat. Sustainability 8, 1339.
6.) Astariz, S., Iglesias, G., 2015. The economics of wave energy: A review. Renewable and Sustainable Energy Reviews 45, 397-408.
7.) Astariz, S., Iglesias, G., 2016. Wave energy vs. other energy sources: A reassessment of the economics. International Journal of Green Energy.
8.) Carballo, R., Iglesias, G., 2013. Wave farm impact based on realistic wave-WEC interaction. Energy 51, 216-229.
9.) Fernandez, H., Iglesias, G., Carballo, R., Castro, A., Fraguela, J., Taveira-Pinto, F., Sanchez, M., 2012. The new wave energy converter WaveCat: Concept and laboratory tests. Marine Structures 29, 58-70.
10.) Iglesias, G., Carballo, R., Castro, A., Fraga, B., Smith, J., 2009. DEVELOPMENT AND DESIGN OF THE WAVECAT (TM) ENERGY CONVERTER. Coastal Engineering 2008, Vols 1-5, 3970-3982.
11.) Veigas, M., Ramos, V., Iglesias, G., 2014. A wave farm for an island: Detailed effects on the nearshore wave climate. Energy 69, 801-812.
Author: Prof Gregorio Iglesias, University of Plymouth (UK)